The energy crisis of the '70s has renewed interest in energy storage devices. The potential of the flywheel has been the subject of a great deal of effort and expense. The main thrust of its development in the United States has been toward flywheel designs using the new synthetic and very strong fibers like E-Glass, carbon fibers, S-Glass, PRD-49 or fused silica in various shapes and designs. (See, for example, "Flywheels", Post et al, Scientific American, December 1973, 17-23; "Perfect Flywheel", Renner-Smith, Popular Science, January 1980, 76-79 and 150; "Mechanical Energy Storage Technology Development Annual Report--FY78 Through Spring 1979", Woods, Sandia Laboratories Report No. SAND79 1151, the disclosures of all being entirely incorporated by reference herein.)
The underlying concept in flywheel storage is the deposition of energy as kinetic energy of rotation and extraction of energy usually via a motor-generator. For efficiency of material and space utilization, however, it is necessary to operate at very high rpm, namely in the area of 30,000 to 50,000 rpm. At such spinning rates, the flywheel is usually situated in vacuum to avoid air friction losses. It must also be supported with high efficiency mechanical or magnetic bearings.
As mentioned above, the primary effort in the development of efficient and inexpensive flywheels has been directed toward the exploitation of the super strong low density modern fibers. The strengths of such fibers permit the use of high rpm's and subsequently high values of the term .sigma./2.rho..
In an unconstrained flywheel, the critical quantity for the performance of a material is the ratio of its tensile strength to its density. The higher this ratio the larger the amount of energy that can be stored as kinetic energy in the flywheel. As a result, metals and in particular steel, which are used for the design of flywheels of moderate energy storage, have all but been abandoned as materials for modern, high rpm, high energy density flywheels. Their strength to density ratio is low and they tend to fail destructively.
On the other hand, the new materials mentioned above have much higher values of this ratio. However, they are also disadvantageous in view of the significant technical and economic problems posed in constructing appropriate flywheel designs from such fibrous materials.
There is a continuing need for improvements and increased latitude in design, preferably, designs which would permit the storage of sufficient energy for practical use and which would increase the maximum amount of storable energy.